Editing Planetary boundary layer (section)

{{Short description|Lowest part of the atmosphere directly influenced by contact with the planetary surface}}
{{Distinguish|planetary boundaries}}
{{Use dmy dates|date=September 2023}}
[[File:The air over Los Angeles.ogv|thumb|300px|This movie is a combined visualization of the PBL and wind dynamics over the Los Angeles basin for a one-month period. Vertical motion of the PBL is represented by the gray "blanket". The height of the PBL is largely driven by [[convection]] associated with the changing surface temperature of the Earth (for example, rising during the day and sinking at night). The colored arrows represent the strength and direction of winds at different altitudes.]]
[[File:PBLimage.jpg|thumb|right|Depiction of where the planetary boundary layer lies on a sunny day.]]

In [[meteorology]], the '''planetary boundary layer''' ('''PBL'''), also known as the '''atmospheric boundary layer''' ('''ABL''') or '''peplosphere''', is the lowest part of the [[atmosphere]] and its behaviour is directly influenced by its contact with a [[planetary surface]].<ref>{{cite web |url=https://www.britannica.com/science/planetary-boundary-layer |title=Planetary boundary layer &#124; atmospheric science &#124; Britannica |website=www.britannica.com |access-date=2020-06-28}}</ref> On Earth it usually responds to changes in surface [[radiative forcing]] in an hour or less. In this layer physical quantities such as [[flow velocity]], temperature, and moisture display rapid fluctuations ([[turbulence]]) and vertical mixing is strong. Above the PBL is the "free atmosphere",<ref>{{cite web |url=http://glossary.ametsoc.org/wiki/Free_atmosphere |title=Free atmosphere |website=glossary.ametsoc.org |access-date=2021-03-21}}</ref> where the wind is approximately [[geostrophic wind|geostrophic]] (parallel to the isobars),<ref>{{cite web |url=http://glossary.ametsoc.org/wiki/Geostrophic_wind_level |title=Geostrophic wind level |website=glossary.ametsoc.org |access-date=2018-09-20}}</ref> while within the PBL the wind is affected by surface [[Drag (physics)|drag]] and turns across the [[Contour line#Barometric pressure|isobar]]s (see [[Ekman layer]] for more detail).